EFFECTS OF FE/C PHASE-SEPARATION ON THE AGES OF WHITE-DWARFS

We have calculated the energy release associated with the phase separation of Fe from C in a predominately C white dwarf. Using an extension of the equation of state developed by Salpeter & Zapolsky, we have computed the total gravitational-plus-internal energy differences between models of homogeneous composition and those with Fe-enriched cores. In the unlikely case where the core is pure Fe, we find a substantial extension of the white dwarf cooling times, even with the small cosmic abundance of this element. For the more realistic core compositions that result if the Fe/C phase diagram is either of the spindle or of the azeotropic type, the energy release is still sufficient to prolong the cooling times by approximately 0.6 Gyr, comparable to that produced by C/O phase separation. We have computed the luminosity function for Fe/C phase separation, as done for C/O by Garcia-Berro et al. Like them, we find that phase separation produces an appreciable "bump" in the luminosity function, although not one large enough to exceed the observational errors at low luminosities. We have also considered both the possibility that C/O, Fe/C, and Ne/C phase separation may occur sequentially, yielding a maximum age approximately 13 Gyr, as well as the possibility that subsequent phase separation processes may be prevented by the first one to occur. The latter case seems the most probable, making the faintest white dwarfs less than 10 Gyr old. It will be necessary to compute phase diagrams for multi-component mixtures and examine their consequences in order to verify this conclusion.